Why Do Snakes Have Pelvic Bones? (Vet Tech Explains)

Why Do Snakes Have Pelvic Bones

Snakes have a reputation for being a little strange. Most of the time, this reputation just isn’t true. Snakes only seem strange because they’re so different from us. Their differences can be explained through evolution and adaptations made for survival.

One interesting fact about snakes is that they still have pelvic bones, even though they don’t use them anymore!

So, why do snakes have pelvic bones?

To put it simply, snakes have pelvic bones because they once had legs. Fossils show that snakes evolved from water-dwelling or landbound reptiles who had front and back legs. Over thousands of years, snakes evolved and lost their legs while still retaining the remnants of a pelvis.

In this article, we’ll talk about why snakes still have their pelvic bones.  We’ll also talk about other vestigial structures snakes have that they no longer need.  But first, let’s define what a vestigial structure is.

What Is A Vestigial Structure?

Vestigial structures, or vestigia, are non-functional features that remain from an earlier evolutionary stage of a species.

You can think of vestigial structures as a remnant of something that once served a purpose. Through a broader definition, vestigial structures might even include things that have a different function from their original function at some point in evolution.

Vestigial structures occur when a species undergoes a significant change in behavior or environment that causes a certain body part to stop being used. Since the function of the structure is no longer necessary for survival, it won’t get passed down to future generations.

Snakes are not the only animals that have vestigial structures. Some great examples of vestigial structures are the wings of flightless birds, human appendixes, and the eyes of cavefish that live in complete darkness. Even though these structures no longer serve a purpose, they are a remnant of something that once did.

The pelvic bones in a snake are a vestigial structure.

Why Do Animals Even Have Pelvic Bones?

Before we talk about why snakes have pelvic bones, let’s talk a little bit about what the pelvis does in general.

The bones of the pelvis are generally a critical part of the skeleton in vertebrates. The pelvis is made up of two large hip bones on each side, made up of the ileum, the ischium, and the pubis bones. Pelvic bones are structurally important for any animals that stand or walk.

The main purpose of pelvic bones in animals (and humans) is to take the weight of the upper body and place it on the lower limbs during movement. This all happens at connections and joints within the pelvis. Pelvic bones also serve to protect reproductive and urinary organs that lie within their area.

Animals have pelvic bones to enable them to walk and stand normally. Even though snakes do have pelvic bones, they still don’t use them to jump, walk, or stand.

Why Do Snakes Have Pelvic Bones?

So, if animals have pelvic bones to walk, why do snakes have them?

As we mentioned above, pelvic bones are vestigial structures in snakes. Snakes no longer use or need their pelvic bones. But, once upon a time, the ancestor of the snakes had four limbs and walked. We know about this evolutionary event because of fossils.

Fossil records show that snakes clearly evolved from some type of reptile, either small burrowing terrestrial lizards or marine reptiles. It’s not easy to get a concrete story of how snakes evolved. This has a lot to do with how fragile their remains are. Even so, snake fossils have been found that date back to as far as the late Jurassic period.

Transitional forms of the lizard to snake specimens have been found and identified. There are actually four separate genera of nearly legless creatures discovered in the Middle East and South America.

Today, we have about 3,000 different species of snakes scattered all around the world. As we know, these snakes don’t have legs and aren’t able to walk in the same way that we do. Snakes have pelvic bones simply as a remnant of the animals that they evolved from.

Do All Snakes Have Pelvic Bones?

Not all snake species are born with pelvic bones. Some have lost them completely through evolution and a lack of need for them.

The species that have vestigial pelvic bones possess bones that are not attached to any vertebra or ribs. So, even though these snakes have backbones, their non-functional pelvic bones aren’t attached to them.

Some snakes, like pythons and boas, have adapted an interesting use for their vestigial pelvic bones. We’ll talk more about that down below!

Why Don’t Snakes Need Pelvises?

If you read the first part of this article, it should be pretty obvious why snakes don’t need pelvises.

Pelvic bones serve several purposes. However, their main function is to bear the weight of the upper body while the lower body is moving. Snakes don’t walk, run, or stand up. Snakes don’t even have legs for the pelvic bones to support! The modern anatomy of a snake has no need for pelvic bones.

Do Snakes Have Other Vestigial Organs Or Structures?

Since snakes evolved from some sort of reptile, it’s natural that they might have more than one vestigial structure. There are a few remnants of past organs that snakes today still have. Here are some of the theories explaining why snakes have these vestiges.

Femurs/Pelvic Spurs

Have you ever heard of pelvic spurs?

Some species of snake have anatomical structures called pelvic spurs. Pelvic spurs are bony claw-like projections found on snakes like boas and pythons. These spurs are found near the cloaca on each side of a snake’s body.

Pelvic spurs have a few different uses but are mainly used for mating purposes. Male snakes will use their pelvic spurs to scratch up and down along a female’s body before meeting. Pelvic spurs are also used during fights to cause damage to other snakes and can sometimes even be useful when climbing!

It’s often believed that pelvic spurs are the remnants of what were once femurs. If that’s a little confusing for you, this video has a great graphic explaining how large femurs became small spurs over time.

Technically, pelvic spurs aren’t a vestigial structure. That’s because they still have a use in the snake’s anatomy today. However, since pelvic spurs aren’t used in the same way that femur bones once were, they’re generally considered to be vestigial.

For more information on pelvic spurs, you can read our article all about it by clicking here.

Pectoral Girdle

Technically, there aren’t any vestigial structures associated with the pectoral girdle found in snakes today. But, things get interesting when we start to look at the muscles in the pectoral area of the snake.

While the general belief is that all of the muscles associated with the pectoral girdle have been lost, some evidence argues otherwise. A study showed that certain hyoid muscles that are associated with the pectoral girdle have been found in some snakes that exist today.

Through this study, we can imagine that though snakes have lost their forelimbs completely, there are still some remnants of the musculature that once helped these animals walk.

Final Thoughts

It’s always surprising to learn about animals that have evolved over time. Ages ago, snakes were reptiles that walked on four legs. Back then, ancestral reptiles had pelvic bones that were fully functional and needed.

Now, snakes don’t have a need for pelvic bones, and yet some of them still have them. This is because these bones are vestigial, having persisted through the evolutionary process. So, even though snakes no longer use their pelvic bones, they can still be found in some of the snake species slithering around the world today.